1. Technical Field
The present invention relates to a heat-sensitive flow rate sensor for measuring flow rate such as amount of intake air of an internal combustion engine. More particularly, the invention relates to an improved heat-sensitive flow rate sensor for measuring velocity or flow rate of a fluid to be measured on the basis of heat transfer phenomenon caused by a heating element or a part heated by the heating element.
2. BACKGROUND ART
FIG. 16 is a plan view showing a flow rate-detecting element in the form of a diaphragm construction used in one of conventional heat-sensitive flow rate sensors. FIG. 17 is a sectional view taken along the line Axe2x80x94A in FIG. 16. In FIGS. 16 and 17, numeral 14 is a flow rate-detecting element. In this flow-rate detecting sensor 14, on the surface of a flat plate-like substrate 1 composed of silicon 0.4 mm thick, an insulating support film 2 composed of silicon nitride of 1 xcexcm in thickness is deposited by spattering, CVD, or any other similar method. Further, on the support film 2, a heating element 3 composed of a heat-sensitive resistance film of platinum, etc. of 0.2 xcexcm in thickness is deposited by evaporation, spattering, or any other similar method. On the heating element 3, patterns serving as current passage are formed by photomechanical process, wet or dry etching, or any other similar method. Further, a fluid-temperature detector 4 composed of a heat-sensitive resistance film of platinum, etc. of 0.2 xcexcmin thickness is formed in the same method as mentioned. Furthermore, on both of the heating element 3 and fluid-temperature detector 4, an insulating protection film 5 composed of silicon nitride, etc. of 1 xcexcm in thickness is deposited by spattering, CVD, or any other similar method. The heating element 3 is connected, through connecting sections 9a, 9b and lead sections 7a, 7d, to electrodes 8a, 8d for electric connection to outside. The fluid temperature-detector 4 is connected through lead sections 7b, 7c to electrodes 8b, 8c. The protection film 5 is removed from the portions of electrodes 8a to 8d to allow electric connection to outside by wire bonding or any other similar method. Further, after forming an etching hole 11 in a backside protection film 10 formed on the opposite face of the support film 2 of the flat plate-like base 1, a cavity 13 being a hollow part is formed by applying alkali etching or the like. Thus, a diaphragm 12 for detecting a flow rate is constructed. An arrow 6 indicates a flowing direction of a fluid to be measured.
Furthermore, as described in the Japanese Patent Publication (unexamined) No. 142020/1998, in case that the flat plate-like flow rate-detecting element 14 of is placed into a fluid to be measured in almost parallel to or at a predetermined angle therefrom, the flow rate-detecting element 14 is placed on the accommodating portion of a supporting member 16, in order to prevent turbulence occurring in the vicinity of the cavity 13, peeling or the like occurring in the front edge portion of the flow rate-detecting element 14. As shown in FIG. 18, the supporting member 16 has a concave accommodating portion 18 to accommodate the flow rate-detecting element 14, and is electrically connected to a detection circuit board through terminals 17 arranged on a base member 20. In FIG. 18, numeral 19 is wires and numeral 21 is a cover. This type of flow rate-detecting element with a diaphragm construction is publicly known, as is also disclosed in the Japanese Patent Publication (unexamined) No. 2967/1992 and others.
FIG. 19 is a front view showing a structure of the heat-sensitive flow rate sensor according to the foregoing prior art, and FIG. 20 is a transverse sectional view taken along the Bxe2x80x94B in FIG. 19. In this conventional heat-sensitive flow rate sensor, a detection pipe passage 100 is placed inside a main passage 101 for a fluid to be measured, and the flow rate-detecting element 14 mounted on the supporting member 16 is placed in the detection pipe passage 100. In FIGS. 19 and 20, numeral 102 is a case for accommodating a detection circuit board 104, numeral 103 is a connector, and numeral 105 is a shield member.
FIG. 21 shows a detection circuit of such conventional heat-sensitive flow rate sensor. The detection circuit board 104 is arranged into a generally used fixed-temperature difference control, and the detection circuit has a bridge circuit including the heating element 3 and the fluid-temperature detector 4. In FIG. 21, R1 to R5 are fixed resistance, OP1 and OP2 are operational amplifiers, TR1 and TR2 are transistors, and BATT is a power supply. The detection circuit, except the heating element 3 and the fluid-temperature detector 4, is arranged on the detection circuit board 4. The detection circuit is driven so as to keep point (a) and point (b) in FIG. 21 at almost the same potential, and controls a heating current IH of the heating element 3. When increasing the velocity of a fluid to be measured, amount of heat transferred from the heating element 3 to the fluid to be measured increases thereby the heating current IH supplied to the heating element 3 being increased. Velocity and flow rate of the fluid to be measured can be obtained by detecting the heating current IH as a voltage Vout at both ends of R3, and such information can be transferred through the connector 103 in FIG. 19 to ECU (electronic control unit).
In the conventional heat-sensitive flow rate sensor of above construction, when mounting the flow rate-detecting element 14 on the accommodating portion 18 in the supporting member 16, the fluid to be measured flows passing through only the surface of the flow rate-detecting element 14 in a small flow rate region. On the other hand, however, in a large flow rate region, a certain amount of the fluid to be measured flows into a gap between the flow rate-detecting element 14 and the accommodating portion 18 in the supporting member 16. Such a flow into the gap is hereinafter referred to as underflow.) Hence, a disadvantage exists in that accuracy in flow rate detection is lowered. FIGS. 22 and 23 show the underflow. In FIGS. 22 and 23, numeral 22 indicates a flow of a fluid to be measured, and numeral 23 indicates a flow of an underflow. To cope with the mentioned disadvantage, for example, the Japanese Patent Publication (unexamined) No. 26343/1997 discloses a structure in which groove-like slots are provided in the accommodating portion of an supporting member along the peripheral edge of a flow rate-detecting element, in order to prevent the underflow produced in the large flow rate region from contacting directly the flow rate-detecting element. Such a structure, however, can not sufficiently prevent the underflow, because the underflow guided by the slots sometimes flows round into the gap formed by the cavity. Furthermore, the Japanese Patent Publication (unexamined) No. 2573/2000 discloses a structure in which one side, either upstream or downstream side, of a flow rate-detecting element is brought into close contact with one side of the accommodating portion in a supporting member in order to prevent the underflow. It is, however, very difficult to bring the side of a flow rate-detecting element into close contact with the side of the accommodating portion. This is because at the time of fixing the flow rate-detecting element to the bottom face of the accommodating portion using some adhesive, the heating element moves slightly as the adhesive is dried. A further problem exists in that it is essential to of connecting the side of the flow rate-detecting element to strictly control dimensional accuracy, surface roughness, etc. of the sides of the accommodating portion and the flow rate-detecting element, which eventually results in low productivity.
The present invention was made to solve the above-discussed problems and has an object of providing a heat-sensitive flow rate sensor that is high in productivity, superior in accuracy of flow rate measurement and sensitivity.
A heat-sensitive flow rate sensor according to the invention comprises: a flow rate-detecting element having a heating element of a heat-sensitive resistance film formed on a surface of a flat plate-like base, and a diaphragm provided with a hollow part at a lower part of the heating element and formed by removing partially a flat plate-like base; and a supporting member having a concave accommodating portion for mounting the flow rate-detecting element thereon and placed inside a pipeline through which a fluid to be measured flows; wherein a protruding face is provided in a raised manner on the bottom face of the accommodating portion of the supporting member so as to block the hollow part on the underside of the flow rate-detecting element.
As a result of such construction, the fluid to be measured can be prevented from flowing into the hollow part, and accuracy in flow rate measurement is improved.
It is preferable that the protruding face blocks entirely the hollow part.
It is also preferable that the protruding face blocks a part of the hollow part while leaving the remaining part open.
As a result of such construction, the fluid to be measured can be restrained from flowing into the hollow part, and the diaphragm can also be prevented from deformation due to expansion or contraction of the fluid to be measured that is enclosed in the hollow part. Thus, accuracy in flow rate measurement is improved.
It is also preferable that the protruding face blocks a part of the hollow part corresponding to a lower part of a pattern of the heating element.
As a result of such construction, amount of heat transferred from the flow rate-detecting element to the supporting member can be restrained, and the fluid to be measured can also be restrained from flowing into the hollow part. Consequently, sensitivity and accuracy in flow rate measurement of the flow rate sensor can be improved.
Another heat-sensitive flow rate sensor according to the invention comprises: a flow rate-detecting element having a heating element of a heat-sensitive resistance film formed on a surface of a flat plate-like base, and a diaphragm formed with a hollow part at a lower part of the heating element and made by removing partially a flat plate-like base; and a supporting member having a concave accommodating portion for mounting the flow rate-detecting element thereon and placed inside a pipeline through which a fluid to be measured flows; wherein a protruding face is provided in a raised manner on the bottom face of the accommodating portion of the supporting member so as to cover a peripheral edge of the hollow part on the underside of the flow rate-detecting element, and the mentioned protruding face has an inclination formed more outside than the external circumference of the hollow part.
As a result of such construction, amount of heat transferred from the flow rate-detecting element to the supporting member can be restrained, and the fluid to be measured can also be restrained from flowing into the hollow part. Thus, sensitivity and accuracy in flow rate measurement of the flow rate sensor can be improved.